Microwave dielectric ceramic composition

ABSTRACT

An object of the present invention is to obtain a dielectric ceramic composition wherein the dielectric constant ∈ is large, the temperature coefficient τf of the resonance frequency is close to 0, and which has a large Q value, by adding to and blending with a ceramic composition whose τf of the resonance frequency is large on the plus side a ceramic composition whose temperature coefficient τf is large on the minus side. In an Li 2 O—R 2 O 3 —TiO 2 -based composition, an improved dielectric constant ∈ can be achieved by introducing a specific quantity of Bi 2 O 3 , and τf can be shifted to the plus side and in addition a considerable improvement in Qf achieved by introducing a specific quantity of MO (where M is one or two of Ca and Sr). Furthermore, by introducing a specific quantity of Na 2 O together with the MO (where M is one or two of Ca and Sr), in particular in the case of material of ∈r&gt;150, it is possible to control τf to the vicinity of 0 while maintaining Qf at an high value.

TECHNICAL FIELD

[0001] The present invention relates to a microwave dielectric ceramiccomposition employed in resonator materials or capacitor materials etcused in the microwave region of a few GHz.

BACKGROUND ART

[0002] Dielectrics are used in the resonators, filters, or capacitorsthat are used in transceivers for for example satellite communicationbroadcasting or mobile identification devices using microwaves of a fewGHz.

[0003] As such dielectric ceramic materials, for exampleBaO—TiO₂—Nd₂O₃—Bi₂O₃ compositions have been proposed (Laid-open JapanesePatent Application No. Sho. 61-8806). The dielectric constant ∈ of thiscomposition is of the order or 70 to 110; when dielectric resonators orcapacitors are constructed, materials of larger dielectric constant ∈are preferred since the larger the dielectric constant ∈ of the materialthat is used, the smaller are the dimensions of the resonator.

[0004] Conventional materials of large dielectric constant ∈ include forexample SrTiO₃ and CaTiO₃ etc; while their dielectric constant ∈ is verylarge at 300 and 180, the temperature coefficient τf of the resonancefrequency is extremely large at +1700 ppm/°C. and +800 ppm/°C., whichmeans that it is not possible to use them.

[0005] Accordingly, as a method of lowering the temperature coefficientτf of a dielectric ceramic composition, the method is available ofpreparing a material whose dielectric constant ∈ is as large as possibleand whose temperature coefficient τf has a negative value; with thismethod, a ceramic composition can be obtained by a suitable compositionwhose dielectric constant ∈ is large and whose temperature coefficientτf of resonance frequency is small.

[0006] For example, Laid-open Japanese Patent Publication No. H.5-211009 proposes the obtaining of a ceramic composition of largedielectric constant ∈ and whose temperature coefficient of resonancefrequency τf is close to zero, by preparing a material represented bythe compositional formula (A¹⁺ _(1/2). B³⁺ _(1/2)) TiO₃ where A¹⁺ isLi¹⁺, and B³⁺ is Nd³⁺, Sm³⁺, Co³⁺ or Pr³⁺, constituting a material oflarge dielectric constant ∈ and wherein the temperature coefficient τfhas a negative value.

[0007] However, the demands for miniaturization of portable electronicterminal equipment have today become so exacting that a material havingeven higher dielectric constant ∈ is earnestly sought for the dielectricmaterial of resonators, filters and capacitors employed in such devices.

DISCLOSURE OF THE INVENTION

[0008] In view of the characteristics required for a dielectric ceramiccomposition for microwaves, an object of the present invention is toobtain a dielectric ceramic composition wherein the dielectric constant∈ is large, the temperature coefficient τf of the resonance frequency isclose to 0, and which has a large Q value, by adding to and blendingwith a ceramic composition whose τf of the resonance frequency is largeon the plus side to a ceramic composition whose temperature coefficientτf is large on the minus side.

[0009] With the object of providing a dielectric ceramic compositionwhose dielectric constant ∈ is large, whose temperature coefficient ofresonance frequency τf is close to 0, and whose Q value is large, thepresent inventors have previously proposed a dielectric ceramiccomposition represented by the compositional formulaLi₂O—Na₂O—Bi₂O₃—R₂O₃—TiO₂, where R₂O₃ is one or two or more of La₂O₃,Nd₂O₃, Sm₂O₃, Co₂O₃ or Pr₂O₃ (Japanese Patent Application Number2000-337141).

[0010] Also, the present inventors have proposed (Japanese PatentApplication Number 2000-340104) a dielectric ceramic compositionrepresented by the compositional formula Li₂O—Bi₂O₃—R₂O₃—SrTiO₃, whereR₂O₃ is one or two or more of La₂O₃, Nd₂O₃, Sm₂O₃, Co₂O₃ or Pr₂O₃.

[0011] As a result of further assiduous investigation, the presentinventors discovered that an excellent dielectric characteristic couldbe obtained in stable fashion by co-presence of specific quantities ofBi₂O₃, Na₂O and MO (where M is one or two of Ca and Sr) in the one ortwo or more of La, Nd, Pr and Sm constituting R, in theLi₂O—R₂O₃—TiO₂-based composition.

[0012] Specifically, the present inventors discovered that, in anLi₂O—R₂O₃—TiO₂-based composition, an improved dielectric constant ∈could be achieved by introducing a specific quantity of Bi₂O₃, and τfcould be shifted to the plus side and in addition the effect of aconsiderable improvement in Qf achieved by introducing a specificquantity of MO (where M is one or two of Ca and Sr).

[0013] They discovered that, by introducing MO, τf becomes too large,making it difficult to make τf approach 0 while maintaining Qf at afixed value, so, by introducing a specific quantity of Na₂O togetherwith the MO (where M is one or two of Ca and Sr), in particular in thecase of material of ∈r>150, it was possible to control τf to thevicinity of 0 while maintaining Qf at an high value, and therebyperfected the present invention.

[0014] Specifically, the present invention consists in a microwavedielectric ceramic composition represented by the compositional formula

aLi₂O-bNa₂O-cR₂O₃-dBi₂O₃-eMO-fTiO₂

[0015] (where a+b+c+d+e+f=100, a, b, c, d, e and f being mol %) where Rincludes one or two or more of Nd, Sm, Pr, and La, M includes one or twoof Ca and Sr and a, b, c, d, e, f satisfy 5<a<15, 0<b<10, 3<c<15,1<d<15, 1<e<30, and 40<f<75.

BEST MODE FOR CARRYING OUT THE INVENTION

[0016] In the present invention, in theLi₂O—Na₂O—R₂O₃—Bi₂O₃—MO—TiO₂-based composition (where R is one or two ormore of Nd, Sm, Pr, and La and M is one or two of Ca and Sr), if Li₂O isless than 5 mol %, the dielectric constant is low and τf becomes toolarge on the plus side; while if it exceeds 15 mol %, Li₂O being of lowmelting-point, this is undesirable because there is the problem that theceramic composition reacts and fuses with the base plate or base powderduring sintering. A range of 5<a<15 mol % is therefore specified forLi₂O (a).

[0017] Also, if Na₂O exceeds 10 mol %, this is undesirable because itgives rise to the problem that τf becomes too large on the plus side,and Qf falls. Accordingly, a range of 0<b<10 mol % is specified for Na₂O(b).

[0018] If R₂O₃ (R³⁺ is one or two or more of Nd³⁺, Sm³⁺, Pr³⁺, and La³⁺)is less than 3 mol %, the dielectric constant is low and Q is also poor;if it exceeds 15 mol %, there is little benefit and costs are increased,which is undesirable. A range of 3<c<15 mol % is therefore specified forR₂O₃ (c).

[0019] Bi₂O₃ has the effect of increasing the dielectric constant ∈ anda content thereof of at least 1 mol % is necessary; however, because ofits lower melting-point, if it exceeds 15 mol %, there is a risk thatthe ceramic composition will react and fuse with the base plate or basepowder on sintering; this is therefore undesirable. Accordingly, a rangeof 1<d<15 mol % is specified for Bi₂O₃ (d).

[0020] Also, the content of MO (where M is one or two of Ca and Sr) hasthe benefit of improving Qf and improving the temperature coefficientτf; however, if it is less than 1 mol %, there is little benefit interms of improving Qf and improving the temperature coefficient τf andif it exceeds 30 mol %, the temperature coefficient τf becomes too largeon the plus side, which is undesirable. Accordingly, a range of 1<e<30mol % is specified for MO (e).

[0021] In addition, if TiO₂ is less than 40 mol %, the requiredcrystalline phase is not obtained and the required dielectriccharacteristic is not obtained; if it is more than 75 mol %, the problemarises that a phase consisting of TiO₂ on its own appears, severelylowering Qf, which is undesirable. A range of 40<f<75 mol % is thereforespecified for TiO₂ (f).

EXAMPLE

[0022] High-purity TiO₂, Bi₂O₃, Li₂CO₃, Nd₂O₃, Sm₂O₃, Pr₆O₁₁, La₂O₃,Na₂CO₃, CaCO₃, and SrCO₃ were weighed out and blended such that therequired mol percentages were obtained in the raw material, as in Table1 to Table 2, so as to obtain a mixture of compositional formulaaLi₂O-bNa₂O-cR₂O₃-dBi₂O₃-eMO-fTiO₂ (where a+b+c+d+e+f=100, a, b, c, d, eand f being mol %, R includes one or two or more of Nd, Sm, Pr, and La,M includes one or two of Ca and Sr); mixing was performed for 5 to 20hours using a ball mill.

[0023] After mixing, calcining was performed for one hour to five hoursat 700° C. to 1000° C. After this, the mixture was again pulverized for2 to 50 hours, and the pulverized powder was mixed with organic binderand pelletized, then sieved and molded into the shape of a disc ofdiameter 10 mm, thickness 6 mm with a pressure of 2 to 3 T/cm². Thismolding was then sintered for one to five hours at 1200° C. to 1400° C.,and both faces thereof polished so that the thickness of the sinteredbody was half its diameter, thereby producing a measurement sample.

[0024] The dielectric constant ∈, Q value and temperature coefficient τfof resonance frequency of the measurement samples obtained were measuredat a measurement frequency of 3 GHz, using the method of Hakki andColeman. The measurement results for the dielectric characteristic ofthe measurement samples are shown in Table 3. TABLE 1 Li₂O Na₂O R₂O₃(R =Na,Sm,Pr,La) (a) (b) (c) 1 9.50 3.97 Sm³⁺ 8.77 2 10.58 4.31 9.03 3 10.005.02 9.57 4 9.70 5.56 9.98 5 10.50 0.38 Nd³⁺ 7.97 6 10.75 0.69 8.24 79.80 0.93 8.44 8 10.70 1.24 8.72 9 8.00 4.01 Pr³ + 0.5 + Nd³ + 0.5 5.8610 9.40 4.35 6.06 11 7.00 5.03 6.45 12 8.00 5.38 6.65 13 9.00 0.30 Pr³⁺6.60 14 9.00 0.60 6.86 15 7.00 0.90 7.11 16 7.00 1.20 7.37 17 8.00 4.14La³ + 0.5 + Nd³ + 0.5 6.85 18 9.00 4.49 7.05 19 9.50 5.19 7.47 20 7.005.56 7.68 1 10.50 0 Nd³⁺ 7.64 2 9.00 0 Pr³⁺ 6.35 3 8.82 0 La³⁺ 4.41 49.50 4.14 Sm³⁺ 13.69 5 10.50 0.30 Nd³⁺ 10.98 6 9.00 0.30 Pr³⁺ 9.17 78.00 8.33 Pr³ + 0.5 + Nd³ + 0.5 8.33 8 9.00 6.67 Pr³⁺ 12.00 9 8.00 6.67La³ + 0.5 + Nd³ + 0.5 8.33

[0025] TABLE 2 Bi₂O₃ MO TiO₂ (d) CaO(e) SrO(e) (f) 1 4.72 0.00 9.5263.52 2 4.86 0.00 8.31 62.90 3 5.15 0.00 5.83 64.43 4 5.38 0.00 3.9265.46 5 3.10 16.79 0.00 61.26 6 3.20 15.67 0.00 61.44 7 3.28 14.81 0.0062.73 8 3.39 13.66 0.00 62.28 9 5.86 14.81 0.00 61.44 10 6.06 13.66 0.0060.48 11 6.45 11.32 0.00 63.75 12 6.65 10.13 0.00 63.20 13 2.57 22.490.00 59.05 14 2.67 21.43 0.00 59.45 15 2.77 20.36 0.00 61.86 16 2.8719.28 0.00 62.28 17 6.85 8.92 0.00 65.25 18 7.05 7.69 0.00 64.72 19 7.475.19 0.00 65.18 20 7.68 3.92 0.00 68.16 1 2.97 18.18 0.00 60.71 2 2.4723.53 0.00 58.85 3 4.41 23.53 0.00 58.82 4 0 0.00 8.92 63.75 5 0 17.070.00 61.15 6 0 22.49 0.00 58.05 7 8.33 0.00 0.00 67.00 8 4.67 0.00 0.0067.67 9 8.33 0.00 0.00 68.67

[0026] TABLE 3 Dielectric characteristic εr Qf(GHz) τf(ppm/° C.) 1 1313202 17 2 133 3134 8 3 135 3198 −2 4 137 3157 −6 5 151 2132 10 6 1532107 3 7 155 2064 −5 8 157 1987 −12 9 167 1767 22 10 170 1723 14 11 1731702 0 12 175 1678 −8 13 179 1545 14 14 182 1512 2 15 185 1448 −4 16 1881398 18 17 198 1312 23 18 202 1280 10 19 208 1223 1 20 217 1187 −5 1 1512209 41 2 180 1533 86 3 205 1109 143 4 111 3256 35 5 129 2389 68 6 1421644 123 7 165 978 4 8 177 745 −5 9 195 465 2

[0027] Industrial Applicability

[0028] In an Li₂O—R₂O₃—TiO₂-based composition, an improved dielectricconstant ∈ can be achieved by introducing a specific quantity of Bi₂O₃,and τf can be shifted to the plus side and in addition the effect of aconsiderable improvement in Qf achieved by introducing a specificquantity of MO (where M is one or two of Ca and Sr). Furthermore, byintroducing a specific quantity of Na₂O together with the MO (where M isone or two of Ca and Sr), in particular in the case of material of∈r>150, it is possible to control τf to the vicinity of 0 whilemaintaining Qf at an high value.

[0029] A dielectric ceramic composition according to the presentinvention is ideal for the dielectric material of resonators, filtersand capacitors used in transceivers for satellite communicationbroadcasting or mobile identification devices etc that make use ofmicrowaves of a few GHz, since, as is clear from the Practical Examples,its dielectric constant ∈ is large, its temperature coefficient τf ofresonance frequency approaches 0, and a large Q value is obtainedtherewith.

1. A microwave dielectric ceramic composition represented by thecompositional formula aLi₂O-bNa₂O-cR₂O₃-dBi₂O₃-eMO-fTiO2 (wherea+b+c+d+e+f=100, a, b, c, d, e and f being mol %), wherein R includesone or two or more of Nd, Sm, Pr, and La, M includes one or two of Caand Sr, and a, b, c, d, e and f satisfy: 5<a<15, 0<b<10, 3<c<15,1<d<15,1<e<30, 40<f<75.